The present invention pertains to a connecting element between ceramic and metallic structural parts, and more especially to such a connecting element which is applied to said parts by thermal, mechanical, electrochemical and also ceramic processes. This connecting element is used, in particular, under conditions of high temperature, aggressive media and high mechanical stresses.
When using ceramic structural parts, it is often necessary to join them to metals. This may be effected by means of metallic bonds such as brazing, welding, etc. or mechanically by all possible types of fastenings, such as, e.g., by key-type pins and fasteners and springs. The requirement posed by practical applications in regard to such joints is high operating safety; however, particularly at elevated temperatures, problems occur. Attempts has been made heretofore to satisfy the abovementioned requirements by the application of suitable metallizing and brazing processes, on the one hand, and by design measures suitable for the materials used, on the other. Metal-ceramic bonds produced in this manner are no longer capable of satisfying the increasing requirements, directed in particular toward higher operating temperatures, reduced wear and extended life of the structural parts.
More recently, therefore, attempts were made to find entirely new ways of accomplishing these bonds. Thus, it is known from U.S. Pat. No. 3,148,981 to use a structural element built up of layers of different compositions, wherein the compositions have increasing contents of metal in the direction of the metal part to be bonded. In composite materials of this type, however, there is always the problem of the incompatibility of certain metal properties in case of a sharp transition from one substance to the other. This is particularly true when the thermal expansion behavior of metals and ceramics is highly different.
Solutions described heretofore, such as metallizing or the use of so-called cermets (i.e. metallic materials with a ceramic matrix or vice versa) are inadequate in certain applications, because the relatively thin connecting zone between the metal and the ceramic does not inhibit the transfer of heat nor does it possess sufficient yield when there are significant differences in the expansion behavior of the structural parts. In addition, different thermal and elastic strains between adjacent structural parts, depending on operating conditions, produce loosening phenomena and overstressing.